1. Field of the Invention
The present invention relates to a molecular pump and, more specifically, to a molecular pump for exhausting or evacuating a chamber or container by the use of a thread groove pump stage.
2. Description of the Related Art
There is a growing need for a pump having a high exhaust capability and being able to achieve a high degree of vacuum with recent accelerated advance in scientific technology.
A molecular pump is widely used in the academic field or in the industrial field as a pump meeting such requirements of the users.
The molecular pump includes a thread groove pump, a turbo molecular pump, or the combination thereof.
FIG. 11 is an illustration of a structure of a molecular pump in the related art, constructed of a turbo molecular pump on the inlet port side and a thread groove pump on the exhaust port side.
A molecular pump 101 includes a turbo molecular pumping stage 102 and a thread groove pumping stage 103. Gas sucked through an inlet port 104 is compressed in the turbo molecular pumping stage 102, and then further compressed in the thread groove pumping stage 103, and finally discharged from an exhaust port 105.
The molecular pump 101 includes a rotor shaft 106, and the rotor shaft 106 is rotatably supported by magnetic bearings 107, 108, 109 about the axis. The magnetic bearings 107, 108 allow magnetic levitation of the rotor shaft 106 in the radial direction and a magnetic bearing 109 allows magnetic levitation of the rotor shaft 106 in the thrust direction.
The rotor shaft 106 includes a motor unit 110 substantially on the axial midsection thereof, and torque generated by the motor unit 110 allows fast axial rotation thereof.
A rotor 111 is secured on the rotor shaft 106 on the side of the inlet port 104 by means of a bolt. The rotor 111 includes a turbine section constituting a body of revolution of the turbo molecular pumping stage 102 and a cylindrical section 122 constituting a body of revolution of the thread groove pumping stage 103.
The turbine section is formed with a number of rotor vanes 112 of multiple stages in the radial direction. A casing 114 is formed with stator vanes 113 of multiple stages on the inner peripheral surface thereof so as to be directed toward the rotor shaft 106 and arranged alternately between the rotor vanes 112.
A thread groove spacer 116 is disposed around the outer peripheral surface of a cylindrical section 122 having a cylindrical outer peripheral surface with a predetermined clearance therefrom. The thread spacer 116 has a cylindrical inner peripheral surface, on which a thread groove 120 is formed in a helical manner.
The molecular pump 101 constructed as described above operates as follows.
After magnetic levitation of the rotor shaft 106 is effected by the magnetic bearings 107, 108, 109, the motor unit 110 is driven to rotate the rotor 111 and gas is sucked through the inlet port 104. Sucked gas is compressed in the turbo molecular pumping stage 102 and fed to the thread groove pumping stage 113 by the action of the rotor vanes 112 and the stator vanes 113. In the thread groove pumping stage 103, gas is guided through the thread groove 120 as a flow path along the cylindrical section 122 rotating at high-velocity, and is further compressed while being carried downwardly. In this manner, gas sucked through the inlet port 104 is compressed in the turbo molecular pumping stage 102, and then further compressed in the thread groove pumping stage 103, and finally discharged from the exhaust port 105.
In this manner, the reason why two types of molecular pump are combined is that the optimal pump differs depending on the pressure range. Accordingly, a molecular pump having a high compression ratio may be realized by constructing the front stage of gas compression of the turbo molecular pumping stage 102 and the rear stage of the thread groove pumping stage 103.
FIG. 12 shows a connecting state between the molecular pump 101 and a chamber 126 in the related art.
When the turbo molecular pump 101 is connected to the chamber 126 to which gas is discharged, the turbo molecular pump 101 may be connected via a gate valve 125. The gate valve 125 is disposed for adjusting the pressure in the chamber 126, and is capable of adjusting the pressure in the chamber 126 by adjusting the opening of the gate valve 125 while operating the turbo molecular pump 101.
However, in the thread groove pumping stage 103 in the related art, a clearance 121 between the rotor 122 and the surface opposed thereto is set to a certain value (for example, 1 mm) or more for ensuring safety and hence preventing the thread groove pumping stage 103 and the rotor 122 from coming into contact. As a result when the gas pressure discharged by the pump is increased, a backflow of gas may characteristically occur through the clearance 121 between the rotor 122 and the surface opposing thereto, which results in lowering of performance.
On the other hand, though there were market requirements to control the pressure by controlling exhaust capability of the pump, the only way was to change the revolution of the rotor 111 in the related art. However, changing the revolution of the rotor is time consuming and, as a result, the pressure of the chamber 126 is controlled by means of the expensive gate valve 125, which results in increase in costs.
Accordingly, it is an object of the invention to provide a molecular pump having a minimum clearance 12, having high gas compressibility, and being capable of controlling gas compressibility.
According to the invention, gas compressibility may be increased, and gas compressibility may be controlled in a molecular pump.
According to the invention, in order to achieve the object described above, there is provided a molecular pump including a stator, a rotor having an opposing surface that faces toward a predetermined surface of the stator and being rotatably supported with the opposing surface faced toward the surface, a motor for driving and rotating the rotor with respect to the stator, a thread groove formed on at least one of the surfaces of the stator and the rotor that face toward each other, a transport device for transporting gas through the thread groove by rotating the rotor by the motor, and a clearance varying device for varying the magnitude of the clearance between the opposing surfaces of the stator and the rotor (first structure).
The first structure may be achieved by providing a device that is capable of varying the magnitude of the clearance between the rotor and the surface opposed thereto as desired at the thread groove section of the thread groove pump or the turbo molecular pump. The clearance varying device allows setting of the magnitude of the clearance by means of a mechanism that moves the rotor or the surface opposing thereto in the axial direction by varying the floating position of the magnetic bearing. A thread groove is formed on at least one of the opposing surfaces of the rotor and the stator, so that gas is transported through the thread groove while being compressed with the rotation of the rotor.
The first structure may be such that the bus line of the surface of the rotor that faces toward the stator forms a predetermined angle, which is larger than zero degree at the smallest, with respect to the axis of the rotor, and the clearance varying device varies the magnitude of the clearance by moving at least one of the rotor and the stator in the direction of the axis of the rotor (second structure).
If the angle formed between the bus line and the axis is zero degree, the opposing surfaces of the rotor and the stator become cylindrical and if it is 90, the opposing surfaces of the rotor and the stator become disk shape. When it is a predetermined angle but not zero degree, the opposing surfaces becomes substantially cylindrical such as the outer peripheral surface of the conical shape, and thus the diameter of the substantially cylindrical shape varies in the axial direction. The rate of change of the diameters of the rotor and of the cylinder opposing thereto, that is, the angle formed between the bus line and the axis may be at least 10.
The second structure may be such that the rotor is rotatably supported by the magnetic bearing, and the clearance varying device moves the rotor in the axial direction by varying the amount of magnetic levitation in the direction of the axis of the rotor effected by the magnetic bearing (third structure).
The second structure may be such that the stator is held by an elastic member being capable of expanding and contracting in the direction of the axis of the rotor, and the clearance varying device moves the stator in the direction of the axis of the rotor by expanding and contracting the elastic member (fourth structure).
The first structure may be such that the outer peripheral surface of the rotor and the inner peripheral surface of the stator are cylindrical and the clearance varying device includes an inner diameter varying device for varying the inner diameter of the inner peripheral surface of the stator (fifth structure).
The clearance varying device may vary the magnitude of the clearance between the opposing surfaces of the rotor and the stator by a mechanism for adjusting the inner diameter of the opposing surface on the stator side.
The fifth structure may be such that the stator includes a stator constituent member divided in the circumferential direction of the inner peripheral surface thereof into a plurality of stator constituent members and an elastic member connecting the divided stator constituent members and being capable of expanding and contracting in the circumferential direction, and the inner diameter varying device varies the inner diameter of the inner peripheral surface of the stator by expanding and contracting the elastic member (sixth structure).
The mechanism for adjusting the inner diameter of the opposing surface on the stator side may be constructed of a cylinder divided into at least two pieces and parts (electrostrictive element) for supporting the same.
The fifth structure may be such that the stator includes the stator constituent member divided into a plurality of members circumferentially of the inner peripheral surface and an elastic member being attached to the outer peripheral surface of the stator constituent member at one end thereof and to the fixed portion at the other end thereof, and being capable of expanding and contracting in the radial direction of the inner peripheral surface, that a clearance is defined between the stator constituent members, and that the inner diameter varying device varies the inner diameter by moving the members radially by expanding and contracting the elastic member (seventh structure)
The fifth structure may be such that the stator is formed with a thread groove on the inner peripheral surface thereof, at least a part of the portion that constitute the thread of the thread groove is formed of the elastic member that is capable of expanding and contracting in the radial direction of the inner peripheral surface thereof, and the inner diameter varying device varies the inner diameter by expanding and contracting the elastic member (eighth structure).
The thread groove is formed on the surface opposing to the rotor (that is, the inner peripheral surface of the stator) and the height of the thread is variable.
Any one of the first to eighth structures may further include a measuring device for measuring the magnitude of the clearance between the rotor and the stator, and an adjusting device for adjusting the magnitude of the clearance by the use of the clearance varying device so that the magnitude of the clearance measured by the measuring device becomes a predetermined value (ninth structure)
The clearance between the opposing surfaces of the rotor and stator may be measured by the (clearance) measuring device such as an eddy current sensor, and controlled by performing feedback control on the margin of clearance based on the output from the measuring device.
It is also applicable to provide a device for measuring the temperature of at least one of the rotor and the surface opposing thereto as the measuring device for measuring the magnitude of the clearance to calculate the magnitude of the clearance based on the output signals therefrom.
Alternatively, the molecular pump that is capable of adjusting the clearance between the opposing surfaces of the rotor and the stator based on signals fed outside, such as the pressure in an exhausted container, and performing feedback control on the performance of the molecular pump, or a vacuum exhaust system using the same may be realized.
Any one of the fourth structure, and the sixth through the ninth structures may be such that the elastic member is formed of an electrostrictive element disposed so as to be capable of applying electric field, and the clearance varying device expands and contracts the electrostrictive element by varying the electric field to be applied on the electrostrictive element (tenth structure).
Any one of the first to the tenth structures may further include a detection device for detecting abnormal circumstance in which the rotor and the stator constituting the molecular pump body may come into contact with each other, and an emergency control device for varying the clearance between the rotor and the stator at least to the extent that is enough for avoiding the contact between them when abnormal condition is detected by the detection device (eleventh structure).
Furthermore, any one of the first to the eleventh structures may a pressure control device for varying the magnitude of the clearance based on the detection signals of the gas pressure in the vacuum container, so that the pressure in the vacuum container may be controlled (twelfth structure).